1. Field of the Invention
The present invention relates to a photovoltaic element having microcrystal semiconductor layers of different light absorption coefficients as an i-type layer of pin type semiconductor layers, and a solar cell module comprised of a plurality of such connected photovoltaic elements.
2. Related Background Art
The photovoltaic elements for converting light to electric energy are commonly applied as solar cells to power supplies for small power in consumer-oriented products, such as desk-top calculators, watches, etc., and are drawing attention as to practical use thereof as future alternate power of the so-called fossil fuels, such as petroleum, coal, and so on. Further, they are also used as sensors in facsimile machines, scanners, and so on.
The photovoltaic elements are elements utilizing the photoelectromotive force (photovoltage) of the pn junction, the Schottky junction, or semiconductors, in which the semiconductor of silicon, or the like absorbs the light to generate photocarriers such as electrons and holes, and the photocarriers drift outside due to an internal electric field of the pn junction part.
The most commonly photovoltaic elements heretofore used single-crystal silicon as a material. Most of such photovoltaic elements are produced by a semiconductor process. Specifically, a single crystal of silicon valency-controlled in the p-type or in the n-type, is prepared by a crystal growth method, such as the CZ method or the like, and the single crystal is sliced into silicon wafers to achieve the thickness of about 300 .mu.m. Further, the pn junction is made by forming layers of different conduction types by appropriate means, such as diffusion of a valency controller to make the conduction type opposite to that of the wafer.
Incidentally, the photovoltaic elements using such single-crystal silicon are increasing their production cost, because the production cost of silicon wafer is high, and photovoltaic elements made using the semiconductor process are expensive. Therefore, the production cost per unit generated power becomes higher than by the existing power generation methods, and that it is considered that it is difficult to decrease to an applicable level for power.
To development practical use of the photovoltaic elements for power, it is thus realized that important technological challenges are to decrease cost and increase area. A search has been conducted for materials and low cost materials, materials with high conversion efficiency, and so on.
Such materials for photovoltaic elements include tetrahedrally bonded amorphous semiconductors, such as amorphous silicon, amorphous silicon germanium, amorphous silicon carbide, and so on, polycrystal semiconductors, compound semiconductors of II-VI group such as CdS, Cu.sub.2 S, or the like, and III-V group such as GaAs, GaAlAs, or the like, and so on. Among others, the thin film photovoltaic elements using the amorphous semiconductors or the polycrystal semiconductors for the photovoltage generating layers are considered to be promising because of the advantage of obtaining a larger-area film, while the thickness of film is reduced, compared to the photovoltaic elements using the single-crystal silicon, and the film can be deposited on an arbitrary support substrate material, and so on.
The above thin film photovoltaic elements, however, have not yet reached the photovoltaic efficiency (photoelectric conversion efficiency) comparable to those of the photovoltaic elements using the single-crystal silicon and, therefore, improvements in the photovoltaic efficiency and in reliability were points to be studied in order to realize the practical use thereof as elements for power.
Recently, A. Shah et al., 23th IEEE Photovoltaic Specialist Conf. (1993) p839, disclosed the technology of solar cells using microcrystalline silicon for the carrier generating layer. It is reported that such solar cells do not suffer the optical degradation phenomenon (Staebler-Wronski effect) specific to the amorphous semiconductors.
On the other hand, Japanese Laid-open Patent Application No. 8-172208 discloses lamination of amorphous semiconductor and single crystals of the crystalline structure, or axially oriented polycrystals, as semiconductor devices of the superlattice structure, in which a plurality of semiconductor substances of different kinds are alternately stacked each in the thickness of about several ten .ANG..
Further, Japanese Patent Publication No. 7-38453 discloses repetitive lamination of microcrystal silicon and amorphous silicon.
The conversion efficiencies of the solar cells reported in A. Shah et al., 23th IEEE Photovoltaic Specialist Conf. (1993) p839 are lower than those of the crystalline silicon solar cells. It is also reported that deposition rates are low.
In general, microcrystal silicon films are made using RF glow discharge, but light absorption of such microcrystal silicon films is low, because they have indirect optical edges, similar to the crystalline silicons. Therefore, the thickness of film needs to be approximately 5 .mu.m, and a great deal of time is necessary for production.
In the above technology of A. Shah et al., 23th IEEE Photovoltaic Specialist Conf. (1993) p839, though the frequency of 70 MHz is used, the film thickness is up to 3 .mu.m and depositing rates are approximately 1 .ANG./sec; therefore, the deposition of film still requires a long time.
The techniques disclosed in Japanese Laid-Open Patent Application No. 8-172208 and Japanese Patent Publication No. 7-38453 are the lamination of amorphous semiconductor and single crystal or polycrystals, or microcrystals, and, because either of them uses the amorphous semiconductor, the optical degradation phenomenon (Staebler-Wronski effect) specific to the amorphous semiconductors cannot be avoided.